A: Making a full-featured media framework is a huge undertaking in itself. By using the work done by others, we both reduce the amount of redundant work being done and leave ourselves free to work on the architecture itself instead of working on the low-level stuff. We would be stupid not to reuse the code others have written.
However, do realize that in no way you are forced to have all dependencies installed. None of the core developers has all of them installed. GStreamer has only a few obligate dependencies : GLib 2.0, liboil, and very common stuff like glibc, a C compiler, and so on. All of the other dependencies are optional.
So, in closing, let's rephrase the question to "Why are you giving me so many choices and such a rich environment ? "
A: Since the 0.3.3 release of GStreamer, we use GLib 2.0 as the core library for GStreamer, which features a move of GObject from GTK+ 2.0 to GLib 2.0. If you want to compile using GTK+ 1.2/GLib 1.2, you need to get the 0.3.1 or earlier release. It is of course not supported.
A: We do have support for the dxr3, although dxr2 support is unknown. GStreamer can easily accomodate hardware acceleration by writing new device-specific elements.
A: Yes, we have no X dependency in any of our core modules. There are GStreamer applications that run fine without any need for X. However, until our Linux Framebuffer or libsvga plugin is ready, you will not be able to play videos without X. In the future, there will probably be lots of different output plugins for video available.
A: GStreamer actively supports such efforts, and in the case of LADSPA, we already have a wrapper plugin. This wrapper plug-in detects the LADSPA plugins present on your system at register time.
A: Not yet. The GStreamer architecture should be able to support the needs of MIDI applications very well however. If you are a developer interested in adding MIDI support to GStreamer we are very interested in getting in touch with you.
A: No. But many of the applications developed for GStreamer do, including our sample applications. There is nothing hindering people from developing applications using other toolkits however and we would happily help promote such efforts. A good example of an application using GStreamer, but which is not using GNOME is the Mozstreamer which uses Mozilla XUL.